Background: The Repeat Expansion Diseases are caused by intergenerational expansions of a specific tandem repeat. More than 20 such diseases have been identified thus far. Expansion of a CGG.CCG-repeat in the 5'UTR of the FMR1 gene is associated with 3 different clinical presentations: Individuals with 55-200 repeats, the so-called premutation allele, are at risk for a neurodegenerative disorder, Fragile X-associated tremor-ataxia syndrome and a form of infertility known as FX-associated primary ovarian insufficiency. Furthermore, in females, the premutation allele can undergo expansion on intergenerational transfer that can result in their children having alleles with >200 repeats. This expanded allele is known as a full mutation and individuals who inherit such alleles almost always have Fragile X syndrome (FXS), which is the leading heritable cause of intellectual disability. The mechanism by which is expansion occurs is unknown. It is thought to differ from the generalized microsatellite instability seen in many different cancers in that the instability is confined to a single genetic locus and shows a strong expansion bias. Expanded alleles are also associated with a folate-sensitive fragile site that is coincident with the repeat on the X chromosome. This site, which gives the disorder its name, is one of many fragile sites present on the human genome. These sites are prone to breakage and in some cases are associated with deleterious chromosome deletions and translocations. Progress report: Tandem repeats are prone to deletion when cloned in bacteria. This makes it difficult to develop contructs to study the biological effect of these sequences. It also results in under-represention of such sequences in databases based on cloned sequences. This can lead to a variety of problems that may impact human health. We have shown that the cloning difficulties can be ameliorated using linear plasmids with low levels of transcription (Godiska et. al., 2009). We have also shown that KBrO3, a potent DNA oxidizing agent that is used in food production and certain hair products and can be found in the water supply as a byproduct of certain sterilization methods, significantly increases the germline expansion frequency (Entezam, Lokanga, Le, Hoffman and Usdin, 2010). Our data thus suggest that oxidative damage, either intrinsic or environmental, may be a factor affecting expansion risk in humans. In parallel experiments we showed that neither a Fen1 nor a Lig1 deficiency had any effect on the expansion frequency, suggesting that delayed Okazaki fragment processing, which plays a role in expansion in yeast, and Long patch base excision repair, which is frequently involved in the repair of oxidative damage, are unlikely to be involved in the repeat expansion seen in mice.